This achievement is tied to the normal progression of the campaign. Below are the weapon and tactical challenges and a description on how to complete them.
Mission 6: Firefly Rain - Max score of 280.
Weapon Challenge:
Efficient Fire - Use a Submachine Gun and kill ten enemies without reloading. Using a SMG with a high capacity mag, a semi-auto trigger, and a suppressor will ensure that you have enough ammo, your shot count will be low, and can remain undetected to complete one of the tactical challenges. Completing this challenge unlocks the PP-19.
Tactical Challenges:
Make Them Count - Complete the mission firing 50 rounds of ammo or less. You will want to utilize the sync-shot as much as possible. If you are in a big firefight, mark enemies with RB and have your fellow Ghosts take them down.
Ghost Escape - After you reach the down cargo plane, you cannot be wounded and have to be healed by a squad mate. If you happen to fail this challenge, you have to restart the entire mission over for another attempt to earn it back. You cannot simply reload the last checkpoint.
On Board Clean - Sabotage the cargo plane in less than 20 minutes. This challenge starts as soon as you gain control of your player for this mission. To keep an eye on the time remaining, simply hit the back button. Completing all tactical challenges will unlock the Smoke Grenade Launcher.
Sometime during the vehicle ride you will see the bomb that was planted on the cargo plane explode and the plane crash. This will earn you the achievement '...Must Come Down'.